Neuron-glial communication mediated by TNF-α and glial activation in dorsal root ganglia in visceral inflammatory hypersensitivity

Reduction of TRPV1 expression on neurons due to downregulation of P2X7R in neonatal rat dorsal root ganglion satellite glial cells under co-culture conditions

BACKGROUND INFORMATION

The purinergic ligand-gated ion channel 7 receptor (P2X7R) is an ATP-gated ion channel that transmits extracellular signals and induces corresponding biological effects, transient receptor potential vanilloid type 1 (TRPV1) is a non-selective cation channel that maintains normal physiological functions; numerous studies showed that P2X7R and TRPV1 are associated with inflammatory reactions.

RESULTS

The effect of P2X7R knockdown in satellite glial cells (SGCs) on neuronal TRPV1 expression under high glucose and high free fat (HGHF) environment was investigated. P2X7 short hairpin RNA (shRNA) was utilized to downregulate P2X7R in SGCs, and treated and untreated SGCs were co-cultured with neuronal cell lines. The expression levels of inflammatory factors and signaling pathways in SGCs and neurons were measured using Western blot analysis, RT-qPCR, immunofluorescence, and enzyme-linked immunosorbent assays. Results suggested that P2X7 shRNA reduced the expression levels of P2X7R protein and mRNA in SGCs surrounding DRG neurons and downregulated the release of tumor necrosis factor-alpha and interleukin-1 beta via the Ca2+/p38 MAPK/NF-κB pathway. Additionally, the downregulation of P2X7R might decrease TRPV1 expression in neurons via the Ca2+/PKC-ɛ/p38 MAPK pathway.

CONCLUSIONS

Reducing P2X7R expression in SCGs in an HGHF environment could decrease neuronal TRPV1 expression via the Ca2+/PKC-ɛ/p38 MAPK pathway.

Activation of EphrinB2/EphB2 signaling in the spine cord alters glia-neuron interactions in mice with visceral hyperalgesia following maternal separation

Background

Sress early in life has been linked to visceral hyperalgesia and associated functional gastrointestinal disorders. In a mouse model of visceral hyperalgesia, we investigated whether the EphB2 receptor and its EphrinB2 ligand in spinal cord contribute to dysregulation of glia-neuron interactions.

Methods

An established mouse model of stress due to maternal separation (MS). Pups were separated from their mothers for 14 days during early development, then analyzed several weeks later in terms of visceral sensitivity based on the abdominal withdrawal reflex score and in terms of expression of c-fos, EphrinB2, EphB2, and phosphorylated MAP kinases (ERK, p38, JNK).

Results

Visceral hyperalgesia due to MS upregulated EphB2, EphrinB2 and c-fos in the spinal cord, and c-fos levels positively correlated with those of EphB2 and EphrinB2. Spinal astrocytes, microglia, and neurons showed upregulation of EphB2, EphrinB2 and phosphorylated MAP kinases. Blocking EphrinB2/EphB2 signaling in MS mice reduced visceral sensitivity, activation of neurons and glia, and phosphorylation of NMDA receptor. Activating EphrinB2/EphB2 signaling in unstressed mice induced visceral hyperalgesia, upregulation of c-fos, and activation of NMDA receptor similar to maternal separation.

Conclusion

The stress of MS during early development may lead to visceral hyperalgesia by upregulating EphrinB2/EphB2 in the spinal cord and thereby altering neuron-glia interactions.

Single-cell transcriptome analysis reveals distinct cell populations in dorsal root ganglia and their potential roles in diabetic peripheral neuropathy

Diabetic peripheral neuropathy (DPN) is a common complication associated with diabetes, and can affect quality of life considerably. Dorsal root ganglion (DRG) plays an important role in the development of DPN. However, the relationship between DRG and the pathogenesis of DPN still lacks a thorough exploration. Besides, a more in-depth understanding of the cell type composition of DRG, and the roles of different cell types in mediating DPN are needed. Here we conducted single-cell RNA-seq (scRNA-seq) for DRG tissues isolated from healthy control and DPN rats. Our results demonstrated DRG includes eight cell-type populations (e.g., neurons, satellite glial cells (SGCs), Schwann cells (SCs), endothelial cells, fibroblasts). In the heterogeneity analyses of cells, six neuron sub-types, three SGC sub-types and three SC sub-types were identified, additionally, biological functions related to cell sub-types were further revealed. Cell communication analysis showed dynamic interactions between neurons, SGCs and SCs. We also found that the aberrantly expressed transcripts in sub-types of neurons, SGCs and SCs with DPN were associated with diabetic neuropathic pain, cell apoptosis, oxidative stress, etc. In conclusion, this study provides a systematic perspective of the cellular composition and interactions of DRG tissues, and suggests that neurons, SGCs and SCs play vital roles in the progression of DPN. Our data may provide a valuable resource for future studies regarding the pathophysiological effect of particular cell type in DPN.

Satellite Glial Cells Bridge Sensory Neuron Crosstalk in Visceral Pain and Cross-Organ Sensitization

Following colonic inflammation, the uninjured bladder afferent neurons are also activated. The mechanisms and pathways underlying this sensory neuron cross-activation (from injured neurons to uninjured neurons) are not fully understood. Colonic and bladder afferent neurons reside in the same spinal segments and are separated by satellite glial cells (SGCs) and extracellular matrix in dorsal root ganglia (DRG). SGCs communicate with sensory neurons in a bidirectional fashion. This review summarizes the differentially regulated genes/proteins in the injured and uninjured DRG neurons and explores the role of SGCs in regulation of sensory neuron crosstalk in visceral cross-organ sensitization. The review also highlights the paracrine pathways in mediating neuron-SGC and SGC-neuron coupling with an emphasis on the neurotrophins and purinergic systems. Finally, I discuss the results from recent RNAseq profiling of SGCs to reveal useful molecular markers for characterization, functional study, and therapeutic targets of SGCs. SIGNIFICANCE STATEMENT: Satellite glial cells (SGCs) are the largest glial subtypes in sensory ganglia and play a critical role in mediating sensory neuron crosstalk, an underlying mechanism in colon-bladder cross-sensitization. Identification of novel and unique molecular markers of SGCs can advance the discovery of therapeutic targets in treatment of chronic pain including visceral pain comorbidity.

Cardiac vagal afferent neurotransmission in health and disease: review and knowledge gaps

The meticulous control of cardiac sympathetic and parasympathetic tone regulates all facets of cardiac function. This precise calibration of cardiac efferent innervation is dependent on sensory information that is relayed from the heart to the central nervous system. The vagus nerve, which contains vagal cardiac afferent fibers, carries sensory information to the brainstem. Vagal afferent signaling has been predominantly shown to increase parasympathetic efferent response and vagal tone. However, cardiac vagal afferent signaling appears to change after cardiac injury, though much remains unknown. Even though subsequent cardiac autonomic imbalance is characterized by sympathoexcitation and parasympathetic dysfunction, it remains unclear if, and to what extent, vagal afferent dysfunction is involved in the development of vagal withdrawal. This review aims to summarize the current understanding of cardiac vagal afferent signaling under in health and in the setting of cardiovascular disease, especially after myocardial infarction, and to highlight the knowledge gaps that remain to be addressed.

Satellite glial cells drive the transition from acute to chronic pain in a rat model of hyperalgesic priming

Chronic pain is one of the most common clinical syndromes affecting patients' quality of life. Regulating the transition from acute to chronic pain is a novel therapeutic strategy for chronic pain that presents a major clinical challenge. However, the mechanism underlying pain transitions remains poorly understood. A rat hyperalgesic priming (HP) model, which mimics pain transition, was established decades ago. Here, this HP model and RNA sequencing (RNA-seq) were used to study the potential role of neuroinflammation in pain transition. In this study, HP model rats developed prolonged hyperalgesia in the hind paw after carrageenan (Car) and PGE2 injection, accompanied by obvious satellite glial cell (SGC) activation in the dorsal root ganglion (DRG), as indicated by upregulation of GFAP. RNA-Seq identified a total of differentially expressed genes in the ipsilateral DRG in HP model rats. The expression of several representative genes was confirmed by real-time quantitative PCR (qPCR). Functional analysis of the differentially expressed genes indicated that genes related to the inflammatory and neuroinflammatory response showed the most significant changes in expression. We further found that the expression of the chemokine CXCL1 was significantly upregulated in the rat DRG. Pharmacological blockade of CXCL1 reduced protein kinase C epsilon overproduction as well as hyperalgesia in HP rats but did not prevent the upregulation of GFAP in the DRG. These results reveal that neuroinflammatory responses are involved in pain transition and may be the source of chronic pain. The chemokine CXCL1 in the DRG is a pivotal contributor to chronic pain and pain transition in HP model rats. Thus, our study provides a putative novel target for the development of effective therapeutics to prevent pain transition.

Special Focus on the Cellular Anti-Inflammatory Effects of Several Micro-Immunotherapy Formulations: Considerations Regarding Intestinal-, Immune-Axis-Related- and Neuronal-Inflammation Contexts

INTRODUCTION

Chronic inflammation is a pernicious underlying status, well-known for its contribution to the progressive development of various diseases. In this regard, Micro-immunotherapy (MI) might be a promising therapeutic strategy. MI employs low doses (LD) and ultra-low doses (ULD) of immune regulators in their formulations. In particular, as both IL-1β and TNF-α are often used at ULD in MI medicines (MIM), a special emphasis has been made on formulations that include these factors in their compositions.

METHODS

Several in vitro models have been employed in order to assess the effects of two unitary MIM consisting of ULD of IL-1β and TNF-α (u-MIM-1 and u-MIM-2, respectively), and four complex MIM (c-MIM-1, -2, -3 and -4) characterized by the presence of ULD of IL-1β and TNF-α amongst other factors. Thus, we first investigated the anti-inflammatory effects of u-MIM-1 and u-MIM-2 in a model of inflamed colon carcinoma cells. In addition, the anti-inflammatory potential of c-MIM-1, -2, -3 and -4, was assessed in in vitro models of intestinal and neuronal inflammation.

RESULTS

The results revealed that u-MIM-1 and u-MIM-2 both induced a slight decrease in the levels of IL-1β and TNF-α transcripts. Regarding the c-MIMs' effects, c-MIM-1 displayed the capability to restore the altered transepithelial electrical resistance in inflamed-HCoEpiC cells. Moreover, c-MIM-1 also slightly increased the expression of the junction-related protein claudin-1, both at the mRNA and protein levels. In addition, our in vitro investigations on c-MIM-2 and c-MIM-3 revealed their immune-modulatory effects in LPS-inflamed human monocytes, macrophages, and granulocytes, on the secretion of cytokines such as TNF-α, PGE2, and IL-6. Finally, c-MIM-4 restored the cell viability of LPS/IFN-γ-inflamed rat cortical neurons, while reducing the secretion of TNF-α in rat glial cells.

DISCUSSION

Our results shed the light on the potential role of these MIM formulations in managing several chronic inflammation-related conditions.

Electroacupuncture Attenuates Post-Inflammatory IBS-Associated Visceral and Somatic Hypersensitivity and Correlates With the Regulatory Mechanism of Epac1-Piezo2 Axis

Electroacupuncture (EA) is considered to have a therapeutic effect in the relief of irritable bowel syndrome (IBS)-associated visceral hypersensitivity via the reduction of the level of 5-hydroxytryptamine (5-HT) and 5-HT₃ receptors (5-HT₃R). However, whether Epac1/Piezo2, as the upstream of 5-HT, is involved in this process remains unclear. We investigated whether EA at the ST36 and ST37 acupoints alleviated visceral and somatic hypersensitivity in a post-inflammatory IBS (PI-IBS) model mice via the Epac1-Piezo2 axis. In this study, we used 2,4,6-trinitrobenzene sulfonic acid (TNBS)-induced PI-IBS as a mouse model. Visceral sensitivity was assessed by the abdominal withdrawal reflex test. Somatic sensitivity was evaluated by the hind paw withdrawal threshold. Quantitative real-time PCR, immunofluorescence staining, ELISA, and Western blotting were performed to examine the expressions of Epac1, Piezo2, 5-HT, and 5-HT₃R from the mouse distal colon/L5-S2 dorsal root ganglia (DRG). Our results showed that EA improved the increased visceral sensation and peripheral mechanical hyperalgesia in PI-IBS model mice, and the effects of EA were superior to the sham EA. EA significantly decreased the protein and mRNA levels of Epac1 and Piezo2, and reduced 5-HT and 5-HT₃R expressions in the distal colon. Knockdown of colonic Piezo2 eliminated the effect of EA on somatic hypersensitivity. Combined knockdown of colonic Epac1 and Piezo2 synergized with EA in relieving visceral hypersensitivity and blocked the effect of EA on somatic hypersensitivity. Additionally, protein levels of Epac1 and Piezo2 were also found to be decreased in the L5-S2 DRGs after EA treatment. Taken together, our study suggested that EA at ST36 and ST37 can alleviate visceral and somatic hypersensitivity in PI-IBS model mice, which is closely related to the regulation of the Epac1-Piezo2 axis.

Preemptive Intravenous Nalbuphine for the Treatment of Post-Operative Visceral Pain: A Multicenter, Double-Blind, Placebo-Controlled, Randomized Clinical Trial

INTRODUCTION

Post-operative visceral pain is common in early postoperative period after laparoscopic surgery. As a kappa opioid receptor agonist, the antinociceptive effects of nalbuphine in visceral pain are consistent across a multitude of experimental conditions irrespective of species. We hypothesized that preemptive nalbuphine can decrease the visceral pain for patients with incisional infiltration of ropivacaine after laparoscopic cholecystectomy.

METHODS

In a multicenter, prospective, double-blind, placebo-controlled, randomized clinical trial, 2094 participants scheduled for laparoscopic cholecystectomy were randomly assigned to receive nalbuphine (Nal group, n = 1029) or placebo (Con group, n = 1027). The Nal group received intravenous nalbuphine 0.2 mg·kg-1 and the Con group received saline in a similar way. The primary endpoint was the effect of nalbuphine on post-operative visceral pain intensity scores within 24 h postoperatively. The total amount of analgesic as well as complications were recorded.

RESULTS

A total of 1934 participants were analyzed. Nalbuphine reduced the visceral pain both at rest (β = - 0.1189, 95% CI - 0.23 to - 0.01, P = 0.037) and movement (β = - 0.1076, 95% CI - 0.21 to - 0.01, P = 0.040) compared with placebo. Patients in the Nal group required less frequent supplemental analgesic administration during the first 24 h after surgery. There were fewer patients in the Nal group who experienced nausea and vomiting (PONV) (P = 0.008).

CONCLUSIONS

Preemptive nalbuphine administered at a dose of 0.2 mg·kg-1 was safe and effective at reducing the postoperative visceral pain and supplemental analgesic use in patients undergoing laparoscopic cholecystectomy.

TRIAL REGISTRATION

Chinese Clinical Trial Registry; ChiCTR1800014379.

Pain sensitivity increases with sleep disturbance under predictable chronic mild stress in mice

Even though it has been well documented that stress can lead to the development of sleep disorders and the intensification of pain, their relationships have not been fully understood. The present study was aimed at investigating the effects of predictable chronic mild stress (PCMS) on sleep–wake states and pain threshold, using the PCMS rearing conditions of mesh wire (MW) and water (W) for 21 days. Exposure to PCMS decreased the amount of non-rapid eye movement (NREM) sleep during the dark phase. Moreover, the chronicity of PCMS decreased slow-wave activity (SWA) during NREM sleep in the MW and W groups in both the light and dark phases. Mechanical and aversively hot thermal hyperalgesia were more intensified in the PCMS groups than the control. Higher plasma corticosterone levels were seen in mice subjected to PCMS, whereas TNF-α expression was found higher in the hypothalamus in the W and the trigeminal ganglion in the MW group. The W group had higher expression levels of IL-6 in the thalamus as well. The PCMS paradigm decreased SWA and may have intensified mechanical and thermal hyperalgesia. The current study also suggests that rearing under PCMS may cause impaired sleep quality and heightened pain sensation to painful mechanical and aversively hot thermal stimuli.

Cisplatin-induced activation and functional modulation of satellite glial cells lead to cytokine-mediated modulation of sensory neuron excitability

Cisplatin plays an essential role in the treatment of various cancers. Cisplatin exhibits high efficacy, but it often leads to severe neurotoxic side effects, such as chemotherapy-induced polyneuropathy (CIPN). The pathophysiology of CIPN is not fully understood. There is increasing evidence for damage to satellite glial cells (SGC) and dorsal root ganglion (DRG) neurons. We investigated the influence of cisplatin on the function of SGCs and the direct influence on DRGs. Satellite glial cells were isolated from DRG and exposed to 0.1, 1, 10, or 100 μM cisplatin for 2 h, 4 h, and 24 h. Using immunocytochemical staining and Western blot analysis, the expression of the glial fibrillary acid protein (GFAP), reactive oxygen species (ROS), and inward rectifier potassium channel 4.1 (K_ir4.1) was determined. An increase in the immune reactivity (IR) and protein levels of GFAP and ROS was measured, and a reduction of IR and protein level of K_ir4.1 was detected. A decrease in these channels' current density was observed using the whole-cell patch-clamp recording. The interleukin-6 (IL-6) and tumor necrosis factor α (TNFα) release of SGCs increased after cisplatin exposure as measured using ELISA, and interleukin-1β (IL-1β) decreased. The SGC-secreted factors in the supernatant after cisplatin treatment led to a modulation of cultured DRG neurons' excitability. Taken together, the modulation and function of different SGC proteins could be linked to a direct impact of cisplatin. Further, SGC-secreted factors influenced the excitability of sensory neurons. Overall, SGCs could be a potential target in preventing and treating chemotherapy-induced neuropathic pain.

Neural control of gut homeostasis

The gut-brain axis is a coordinated communication system that not only maintains homeostasis, but significantly influences higher cognitive functions and emotions, as well as neurological and behavioral disorders. Among the large populations of sensory and motor neurons that innervate the gut, insights into the function of primary afferent nociceptors, whose cell bodies reside in the dorsal root ganglia and nodose ganglia, have revealed their multiple crosstalk with several cell types within the gut wall, including epithelial, vascular, and immune cells. These bidirectional communications have immunoregulatory functions, control host response to pathogens, and modulate sensations associated with gastrointestinal disorders, through activation of immune cells and glia in the peripheral and central nervous system, respectively. Here, we will review the cellular and neurochemical basis of these interactions at the periphery, in dorsal root ganglia, and in the spinal cord. We will discuss the research gaps that should be addressed to get a better understanding of the multifunctional role of sensory neurons in maintaining gut homeostasis and regulating visceral sensitivity.

Spinal neuron-glia-immune interaction in cross-organ sensitization

Inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS), historically considered as regional gastrointestinal disorders with heightened colonic sensitivity, are increasingly recognized to have concurrent dysfunction of other visceral and somatic organs, such as urinary bladder hyperactivity, leg pain, and skin hypersensitivity. The interorgan sensory cross talk is, at large, termed "cross-organ sensitization." These organs, anatomically distant from one another, physiologically interlock through projecting their sensory information into dorsal root ganglia (DRG) and then the spinal cord for integrative processing. The fundamental question of how sensitization of colonic afferent neurons conveys nociceptive information to activate primary afferents that innervate distant organs remains ambiguous. In DRG, primary afferent neurons are surrounded by satellite glial cells (SGCs) and macrophage accumulation in response to signals of injury to form a neuron-glia-macrophage triad. Astrocytes and microglia are major resident nonneuronal cells in the spinal cord to interact, physically and chemically, with sensory synapses. Cumulative evidence gathered so far indicate the indispensable roles of paracrine/autocrine interactions among neurons, glial cells, and immune cells in sensory cross-activation. Dichotomizing afferents, sensory convergency in the spinal cord, spinal nerve comingling, and extensive sprouting of central axons of primary afferents each has significant roles in the process of cross-organ sensitization; however, more results are required to explain their functional contributions. DRG that are located outside the blood-brain barrier and reside upstream in the cascade of sensory flow from one organ to the other in cross-organ sensitization could be safer therapeutic targets to produce less central adverse effects.

Satellite glia as a critical component of diabetic neuropathy: Role of lipocalin-2 and pyruvate dehydrogenase kinase-2 axis in the dorsal root ganglion

Diabetic peripheral neuropathy (DPN) is a common complication of uncontrolled diabetes. The pathogenesis of DPN is associated with chronic inflammation in dorsal root ganglion (DRG), eventually causing structural and functional changes. Studies on DPN have primarily focused on neuronal component, and there is limited knowledge about the role of satellite glial cells (SGCs), although they completely enclose neuronal soma in DRG. Lipocalin-2 (LCN2) is a pro-inflammatory acute-phase protein found in high levels in diverse neuroinflammatory and metabolic disorders. In diabetic DRG, the expression of LCN2 was increased exclusively in the SGCs. This upregulation of LCN2 in SGCs correlated with increased inflammatory responses in DRG and sciatic nerve. Furthermore, diabetes-induced inflammation and morphological changes in DRG, as well as sciatic nerve, were attenuated in Lcn2 knockout (KO) mice. Lcn2 gene ablation also ameliorated neuropathy phenotype as determined by nerve conduction velocity and intraepidermal nerve fiber density. Mechanistically, studies using specific gene KO mice, adenovirus-mediated gene overexpression strategy, and primary cultures of DRG SGCs and neurons have demonstrated that LCN2 enhances the expression of mitochondrial gate-keeping regulator pyruvate dehydrogenase kinase-2 (PDK2) through PPARβ/δ, thereby inhibiting pyruvate dehydrogenase activity and increasing production of glycolytic end product lactic acid in DRG SGCs and neurons of diabetic mice. Collectively, our findings reveal a crucial role of glial LCN2-PPARβ/δ-PDK2-lactic acid axis in progression of DPN. Our results establish a link between pro-inflammatory LCN2 and glycolytic PDK2 in DRG SGCs and neurons and propose a novel glia-based mechanism and drug target for therapy of DPN. MAIN POINTS: Diabetes upregulates LCN2 in satellite glia, which in turn increases pyruvate dehydrogenase kinase-2 (PDK2) expression and lactic acid production in dorsal root ganglia (DRG). Glial LCN2-PDK2-lactic acid axis in DRG plays a crucial role in the pathogenesis of diabetic neuropathy.

Emerging importance of satellite glia in nervous system function and dysfunction

Satellite glial cells (SGCs) closely envelop cell bodies of neurons in sensory, sympathetic and parasympathetic ganglia. This unique organization is not found elsewhere in the nervous system. SGCs in sensory ganglia are activated by numerous types of nerve injury and inflammation. The activation includes upregulation of glial fibrillary acidic protein, stronger gap junction-mediated SGC-SGC and neuron-SGC coupling, increased sensitivity to ATP, downregulation of Kir4.1 potassium channels and increased cytokine synthesis and release. There is evidence that these changes in SGCs contribute to chronic pain by augmenting neuronal activity and that these changes are consistent in various rodent pain models and likely also in human pain. Therefore, understanding these changes and the resulting abnormal interactions of SGCs with sensory neurons could provide a mechanistic approach that might be exploited therapeutically in alleviation and prevention of pain. We describe how SGCs are altered in rodent models of four common types of pain: systemic inflammation (sickness behaviour), post-surgical pain, diabetic neuropathic pain and post-herpetic pain.

Clinical Response and Changes of Cytokines and Zonulin Levels in Patients with Diarrhoea-Predominant Irritable Bowel Syndrome Treated with Bifidobacterium Longum ES1 for 8 or 12 Weeks: A Preliminary Report

Bifidobacterium longum (B. longum) ES1 is a probiotic strain capable of modulating microbiome composition, anti-inflammatory activity and intestinal barrier function. We investigated the use of B. Longum ES1 in the treatment of patients with diarrhoea-predominant irritable bowel syndrome (IBS-D). Sixteen patients were treated for 8 or 12 weeks with B. Longum ES1 (1 × 10⁹ CFU/day). Serum zonulin and cytokines were measured at baseline (T0) and at the end of therapy (T1). Clinical response to therapy was assessed by IBS Severity Scoring System. Interleukin (IL)-6, IL-8, IL-12p70 and tumor necrosis factor (TNF) α levels decreased from T0 to T1, irrespective of treatment duration (p < 0.05), while zonulin levels diminished only in patients treated for 12 weeks (p = 0.036). Clinical response was observed in 5/16 patients (31%): 4/8 (50%) treated for 12 weeks and 1/8 (13%) treated for 8 weeks. Abdominal pain improved only in patients treated for 12 weeks (5/8 vs. 0/8, p = 0.025), while stool consistency improved regardless of therapy duration (p < 0.001). In conclusion, the results of this pilot study showed, in IBS-D patients treated for 12 weeks with B. longum ES1, a reduction in the levels of pro-inflammatory cytokines, and intestinal permeability as well as an improvement in gastrointestinal symptoms, but further studies including a placebo-control group are necessary to prove a causal link.

Activation and functional modulation of satellite glial cells by oxaliplatin lead to hyperexcitability of sensory neurons in vitro

Platinum-based chemotherapeutics still play an important role in cancer therapy, however, severe side effects, such as painful neuropathy, occur frequently. The pathophysiologic mechanisms depend on the applied chemotherapeutic agent and are still controversial. In addition to neuronal damage, disturbance of glial cell activity may contribute to neurotoxicity. Here, we focused on the effect of oxaliplatin on satellite glial cell (SGC) function and on the activity of the dorsal root ganglion (DRG) neurons. SGCs were isolated as high-purity cultures and treated with 1 and 10 μM oxaliplatin for 2, 4 and 24 h. Subsequently, glial fibrillary acid protein (GFAP), reactive oxygen species (ROS), Connexin-43 (Cx-43), and inward rectifier potassium channel 4.1 (K_ir4.1) expression was determined by immunocytochemical staining (ICC) and Western blot analyses. Immunochemical staining and Western blot analysis showed an increase in the immune reactivity (IR) and protein levels of ROS, GFAP, and Cx-43. Furthermore, reduction of the IR and protein levels and current density were demonstrated using patch-clamp measurements, of K_ir4.1 channels after oxaliplatin exposure. Cytokine release in SGCs was measured using enzyme-linked immunosorbent assays (ELISA) after oxaliplatin exposure and indicated an increased release of IL-6 and TNFα, while IL-1β was decreased. The direct influence of SGC-secreted factors in the supernatant after oxaliplatin treatment led to the hyperexcitability of cultured DRG neurons. In summary, oxaliplatin has a direct impact on the modulation and function of different SGC proteins. Furthermore, SGC-released factors influence the excitability of sensory neurons, qualifying SGCs as potential targets for the prevention and treatment of oxaliplatin-induced polyneuropathy.

Activated spinal astrocytes contribute to the later phase of carrageenan-induced prostatitis pain

Background

Prostatodynia is the main symptom of chronic prostatitis and the main reason that patients go to the hospital for treatment. Although a variety of factors, including inflammatory immune response, nervous system sensitization, and psychological factors, have been shown to play important roles in the induction and development of chronic pain in prostatitis, the underlying cause of chronic prostatodynia maintenance in prostatitis patients remains unclear.

Methods

A mouse model of chronic prostatitis induced by carrageenan injection was used. The von Frey test was used to measure pain behavior. The microglial and astrocyte activations were immunohistochemically demonstrated with antibodies against Iba1 and GFAP. The expression of cytokine or signaling pathway was detected by enzyme-linked immunosorbent assay (ELISA) and Western blotting.

Results

In this study, we provide several lines of evidence to demonstrate that activated spinal astrocytes contribute to the later phase (5 weeks after carrageenan injection) of carrageenan-induced prostatitis pain. First, activation of spinal astrocytes but not microglia was found in the spinal cord dorsal horn at 5 weeks. Second, intrathecal injection of the astroglial toxin L-2-Aminoadipate acid (L-AA) but not microglial inhibitor minocycline reduced mechanical allodynia at 5 weeks. Third, chronic prostatitis induced a profound and persistent upregulation of connexin-43 hemichannels in spinal astrocytes, and spinal injection of the connexin-43 inhibitor carbenoxolone (CBX) effectively reduced pain symptoms. Fourth, increased expression and release of chemokine C-X-C motif ligand 1 (CXCL1) in the spinal dorsal horn and intrathecal injection of a CXCL1 neutralizing antibody or the CXCR2 (a major receptor of CXCL1) antagonist SB225002 significantly reduced mechanical allodynia at 5 weeks.

Conclusions

In this study, we found that a novel mechanism of activated spinal astrocytes plays a crucial role in maintaining chronic prostatitis-induced persistent pain via connexin-43-regulated CXCL1 production and secretion.

Marine Toxins and Nociception: Potential Therapeutic Use in the Treatment of Visceral Pain Associated with Gastrointestinal Disorders

Visceral pain, of which the pathogenic basis is currently largely unknown, is a hallmark symptom of both functional disorders, such as irritable bowel syndrome, and inflammatory bowel disease. Intrinsic sensory neurons in the enteric nervous system and afferent sensory neurons of the dorsal root ganglia, connecting with the central nervous system, represent the primary neuronal pathways transducing gut visceral pain. Current pharmacological therapies have several limitations, owing to their partial efficacy and the generation of severe adverse effects. Numerous cellular targets of visceral nociception have been recognized, including, among others, channels (i.e., voltage-gated sodium channels, VGSCs, voltage-gated calcium channels, VGCCs, Transient Receptor Potential, TRP, and Acid-sensing ion channels, ASICs) and neurotransmitter pathways (i.e., GABAergic pathways), which represent attractive targets for the discovery of novel drugs. Natural biologically active compounds, such as marine toxins, able to bind with high affinity and selectivity to different visceral pain molecular mediators, may represent a useful tool (1) to improve our knowledge of the physiological and pathological relevance of each nociceptive target, and (2) to discover therapeutically valuable molecules. In this review we report the most recent literature describing the effects of marine toxin on gastrointestinal visceral pain pathways and the possible clinical implications in the treatment of chronic pain associated with gut diseases.

Preemptive oxycodone is superior to equal dose of sufentanil to reduce visceral pain and inflammatory markers after surgery: a randomized controlled trail

Background

Postoperative visceral pain is common after surgery and previous studies have demonstrated that oxycodone is an effective treatment. In this study, we compared the effects of preemptive oxycodone to equal dose of sufentanil on postoperative pain and serum level of inflammatory factors (TNF-α, IL-6, IL-10) after laparoscopic cholecystectomy.

Methods

Forty patients undergoing laparoscopic cholecystectomy were randomized into preemptive oxycodone group or preemptive sufentanil group.

Patients were given either oxycodone 0.1 mg/kg (oxycodone group, n = 20) or sufentanil 0.1 μg/kg (sufentanil group, n = 20) for preemptive analgesia. We evaluated pain/sedation scores at 0 h, 0.5 h, 2 h, 4 h, 6 h, 8 h and 24 h after surgery and measured serum concentrations of TNF-α, IL-6 and IL-10 before surgery and at 0 h, 6 h and 24 h after surgery.

Results

Twenty patients were recruited in each group. Numerical rating scale (NRS) of visceral pain in the oxycodone group at 2 h when resting (0.5(0,2.75) vs 3(2,4), P = 0.008) and moving (0.5(0,3) vs 3(2.25,4), P = 0.015) and 4 h when moving (2(0,3) vs 3(0,4.75), P = 0.043) after surgery were significantly lower than the sufentanil group. Serum concentrations of TNF-α at 6 h (38.68 ± 10.49 vs 73.02 ± 16.27, P<0.001) and 24 h (43.12 ± 8.40 vs 74.00 ± 21.30, P<0.001) in the oxycodone group were lower than the sufentanil group.

Conclusions

Preemptive oxycodone 0.1 mg/kg administration could effectively suppress visceral pain at 2 h and 4 h after surgery and had lower inflammatory marker, serum TNF-α, level when compared to equal dose of sufentanil.

Trial registration

Clinical trials registration number: ChiCTR-IOR-17013738http://www.chictr.org.cn/showproj.aspx?proj=17346. Date of registration: 6th December 2017.

Nitric oxide as a messenger between neurons and satellite glial cells in dorsal root ganglia

Abnormal neuronal activity in sensory ganglia contributes to chronic pain. There is evidence that signals can spread between cells in these ganglia, which may contribute to this activity. Satellite glial cells (SGCs) in sensory ganglia undergo activation following peripheral injury and participate in cellular communication via gap junctions and chemical signaling. Nitric oxide (NO) is released from neurons in dorsal root ganglia (DRG) and induces cyclic GMP (cGMP) production in SCGs, but its role in SGC activation and neuronal excitability has not been explored. It was previously reported that induction of intestinal inflammation with dinitrobenzoate sulfonate (DNBS) increased gap junctional communications among SGCs, which contributed to neuronal excitability and pain. Here we show that DNBS induced SGC activation in mouse DRG, as assayed by glial fibrillary acidic protein upregulation. DNBS also upregulated cGMP level in SGCs, consistent with NO production. In vitro studies on intact ganglia from DNBS-treated mice showed that blocking NO synthesis inhibited both SGCs activation and cGMP upregulation, indicating an ongoing NO production. Application of NO donor in vitro induced SGC activation, augmented gap junctional communications, and raised neuronal excitability, as assessed by electrical recordings. The cGMP analog 8-Br-cGMP mimicked these actions, confirming the role of the NO-cGMP pathway in intraganglionic communications. NO also augmented Ca²⁺ waves propagation in DRG cultures. It is proposed that NO synthesis in DRG neurons increases after peripheral inflammation and that NO induces SGC activation, which in turn contributes to neuronal hyperexcitability. Thus, NO plays a major role in neuron-SGC communication.

Enteric Glia: A New Player in Abdominal Pain

Chronic abdominal pain is the most common gastrointestinal issue and contributes to the pathophysiology of functional bowel disorders and inflammatory bowel disease. Current theories suggest that neuronal plasticity and broad alterations along the brain-gut axis contribute to the development of chronic abdominal pain, but the specific mechanisms involved in chronic abdominal pain remain incompletely understood. Accumulating evidence implicates glial cells in the development and maintenance of chronic pain. Astrocytes and microglia in the central nervous system and satellite glia in dorsal root ganglia contribute to chronic pain states through reactive gliosis, the modification of glial networks, and the synthesis and release of neuromodulators. In addition, new data suggest that enteric glia, a unique type of peripheral glia found within the enteric nervous system, have the potential to modify visceral perception through interactions with neurons and immune cells. Understanding these emerging roles of enteric glia is important to fully understand the mechanisms that drive chronic pain and to identify novel therapeutic targets. In this review, we discuss enteric glial cell signaling mechanisms that have the potential to influence chronic abdominal pain.

Neurons and satellite glial cells in adult rat lumbar dorsal root ganglia express connexin 36

Previous studies have shown that following peripheral nerve injury there was a downregulation of the gap junction protein connexin 36 (Cx36) in the spinal cord; however, it is not known whether Cx36 protein is expressed in the dorsal root ganglia (DRGs), nor if its levels are altered following peripheral nerve injuries. Here we address these aspects in the adult rat lumbar DRG. Cx36 mRNA was detected using qRT-PCR, and Cx36 protein was identified in DRG sections using immunohistochemistry (IHC) and immunofluorescence (IF). Double staining revealed that Cx36 co-localizes with both anti-β-III tubulin, a neuronal marker, and anti-glutamine synthetase, a satellite glial cell (SGC) marker. In neurons, Cx36 staining was mostly uniform in somata and fibers of all sizes and its intensity increased at the cell membranes. This labeling pattern was in contrast with Cx36 IF dots mainly found at junctional membranes in islet beta cells used as a control tissue. Co-staining with anti-Cx43 and anti-Cx36 showed that whereas mostly uniform staining of Cx36 was found throughout neurons and SGCs, Cx43 IF puncta were localized to SGCs. Cx36 mRNA was expressed in normal lumbar DRG, and it was significantly down-regulated in L4 DRG of rats that underwent sciatic nerve injury resulting in persistent hypersensitivity. Collectively, these findings demonstrated that neurons and SGCs express Cx36 protein in normal DRG, and suggested that perturbation of Cx36 levels may contribute to chronic neuropathic pain resulting from a peripheral nerve injury.

Critical Evaluation of Animal Models of Gastrointestinal Disorders

Preclinical research remains an important tool for discovery and validation of novel therapeutics for gastrointestinal disorders. While in vitro assays can be used to verify receptor-ligand interactions and test for structural activity of new compounds, only whole-animal studies can demonstrate drug efficacy within the gastrointestinal system. Most major gastrointestinal disorders have been modeled in animals; however the translational relevance of each model is not equal. The purpose of this chapter is to provide a critical evaluation of common animal models that are being used to develop pharmaceuticals for gastrointestinal disorders. For brevity, the models are presented for upper gastrointestinal disorders involving the esophagus, stomach, and small intestine and lower gastrointestinal disorders that focus on the colon. Particular emphasis is used to explain the face and construct validity of each model, and the limitations of each model, including data interpretation, are highlighted. This chapter does not evaluate models that rely on surgical or other non-pharmacological interventions for treatment.

Tumor necrosis factor-Alpha stimulates cytokine expression and transient sensitization of trigeminal nociceptive neurons

OBJECTIVE

Elevated levels of tumor necrosis factor- alpha (TNF-α) in the capsule of the temporomandibular joint (TMJ) are implicated in the underlying pathology of temporomandibular disorders (TMD). TMD are a group of conditions that result in pain in the TMJ and/or muscles of mastication, and are associated with significant social and economic burdens. The goal of this study was to investigate the effect of elevated TNF-α levels in the TMJ capsule on nocifensive behavioral response to mechanical stimulation of trigeminal neurons and regulation of cytokines within the trigeminal ganglion.

DESIGN

Male Sprague-Dawley rats were injected bilaterally in the TMJ capsule with TNF-α and changes in nocifensive head withdrawal responses to mechanical stimulation of cutaneous tissue directly over the capsule was determined using von Frey filaments. Cytokine levels in trigeminal ganglia were determined by protein array analysis at several time points post injection and correlated to nocifensive behavior.

RESULTS

TNF-α caused a significant increase in the average number of nocifensive responses when compared to naive and vehicle treated animals 2h post injection, but levels returned to control levels at 24h. Based on array analysis, the levels of eight cytokines were significantly elevated above vehicle control levels at 2h following TNF-α injection, but all eight had returned to the vehicle control levels after 24h.

CONCLUSIONS

Our findings provide evidence that elevated levels of TNF-α in the joint capsule, which is reported to occur in TMD, promotes nociception in trigeminal ganglia neurons via a mechanism that temporally correlates with differential regulation of several cytokines.

The Role of Visceral Hypersensitivity in Irritable Bowel Syndrome: Pharmacological Targets and Novel Treatments

Irritable bowel syndrome (IBS) is the most common disorder referred to gastroenterologists and is characterized by altered bowel habits, abdominal pain, and bloating. Visceral hypersensitivity (VH) is a multifactorial process that may occur within the peripheral or central nervous systems and plays a principal role in the etiology of IBS symptoms. The pharmacological studies on selective drugs based on targeting specific ligands can provide novel therapies for modulation of persistent visceral hyperalgesia. The current paper reviews the cellular and molecular mechanisms underlying therapeutic targeting for providing future drugs to protect or treat visceroperception and pain sensitization in IBS patients. There are a wide range of mediators and receptors participating in visceral pain perception amongst which substances targeting afferent receptors are attractive sources of novel drugs. Novel therapeutic targets for the management of VH include compounds which alter gut-brain pathways and local neuroimmune pathways. Molecular mediators and receptors participating in pain perception and visceroperception include histamine-1 receptors, serotonin (5-hydrodytryptamine) receptors, transient receptor potential vanilloid type I, tachykinins ligands, opioid receptors, voltage-gated channels, tyrosine receptor kinase receptors, protease-activated receptors, adrenergic system ligands, cannabinoid receptors, sex hormones, and glutamate receptors which are discussed in the current review. Moreover, several plant-derived natural compounds with potential to alleviate VH in IBS have been highlighted. VH has an important role in the pathology and severity of complications in IBS. Therefore, managing VH can remarkably modulate the symptoms of IBS. More preclinical and clinical investigations are needed to provide efficacious and targeted medicines for the management of VH.

Glial contributions to visceral pain: implications for disease etiology and the female predominance of persistent pain

In the central nervous system, bidirectional signaling between glial cells and neurons ('neuroimmune communication') facilitates the development of persistent pain. Spinal glia can contribute to heightened pain states by a prolonged release of neurokine signals that sensitize adjacent centrally projecting neurons. Although many persistent pain conditions are disproportionately common in females, whether specific neuroimmune mechanisms lead to this increased susceptibility remains unclear. This review summarizes the major known contributions of glia and neuroimmune interactions in pain, which has been determined principally in male rodents and in the context of somatic pain conditions. It is then postulated that studying neuroimmune interactions involved in pain attributed to visceral diseases common to females may offer a more suitable avenue for investigating unique mechanisms involved in female pain. Further, we discuss the potential for primed spinal glia and subsequent neurogenic inflammation as a contributing factor in the development of peripheral inflammation, therefore, representing a predisposing factor for females in developing a high percentage of such persistent pain conditions.

The antinociceptive effect and mechanism of action of SY0916

Pain greatly affects the quality of life of people worldwide. Despite their demonstrated efficacy, currently used opioid drugs and nonsteroidal anti-inflammatory drugs (NSAIDs) are frequently associated with several adverse events. The identification of new therapeutic targets and the development of corresponding analgesics may represent novel approaches for effectively treating pain. SY0916 is a novel compound that was designed and synthesized by the Institute of Materia Medica, Chinese Academy of Medical Sciences. As demonstrated by the hot plate test, tail-flick test and the formalin test, SY0916 exerted strong peripheral and central antinociceptive effects. Western blot, immunohistochemistry and enzyme-linked immunosorbent assay (ELISA) results indicate that SY0916 induces its peripheral antinociceptive effect by suppressing the peripheral activity of inflammatory mediators such as prostaglandin E2 (PGE2), tumor necrosis factor-alpha (TNF-α) and 5-hydroxytryptamine (5-HT). Moreover, its central antinociceptive effect might be mediated by the down-regulation of PGE2 and TNF-α expression and the inhibition of p-p38 and NF-κB pathway signaling in glial cells. These findings demonstrate that SY0916 may serve as a promising analgesic candidate drug.

Functional polarization of neuroglia: Implications in neuroinflammation and neurological disorders

Recent neuroscience research has established the adult brain as a dynamic organ having a unique ability to undergo changes with time. Neuroglia, especially microglia and astrocytes, provide dynamicity to the brain. Activation of these glial cells is a major component of the neuroinflammatory responses underlying brain injury and neurodegeneration. Glial cells execute functional reaction programs in response to diverse microenvironmental signals manifested by neuropathological conditions. Activated microglia exist along a continuum of two functional states of polarization namely M1-type (classical/proinflammatory activation) and M2-type (alternative/anti-inflammatory activation) as in macrophages. The balance between classically and alternatively activated microglial phenotypes influences disease progression in the CNS. The classically activated state of microglia drives the neuroinflammatory response and mediates the detrimental effects on neurons, whereas in their alternative activation state, which is apparently a beneficial activation state, the microglia play a crucial role in tissue maintenance and repair. Likewise, in response to immune or inflammatory microenvironments astrocytes also adopt neurotoxic or neuroprotective phenotypes. Reactive astrocytes exhibit two distinctive functional phenotypes defined by pro- or anti-inflammatory gene expression profile. In this review, we have thoroughly covered recent advances in the understanding of the functional polarization of brain and peripheral glia and its implications in neuroinflammation and neurological disorders. The identifiable phenotypes adopted by neuroglia in response to specific insult or injury can be exploited as promising diagnostic markers of neuroinflammatory diseases. Furthermore, harnessing the beneficial effects of the polarized glia could undoubtedly pave the way for the formulation of novel glia-based therapeutic strategies for diverse neurological disorders.

Role of satellite glial cells in gastrointestinal pain

Gastrointestinal (GI) pain is a common clinical problem, for which effective therapy is quite limited. Sensations from the GI tract, including pain, are mediated largely by neurons in the dorsal root ganglia (DRG), and to a smaller extent by vagal afferents emerging from neurons in the nodose/jugular ganglia. Neurons in rodent DRG become hyperexcitable in models of GI pain (e.g., gastric or colonic inflammation), and can serve as a source for chronic pain. Glial cells are another element in the pain signaling pathways, and there is evidence that spinal glial cells (microglia and astrocytes) undergo activation (gliosis) in various pain models and contribute to pain. Recently it was found that satellite glial cells (SGCs), the main type of glial cells in sensory ganglia, might also contribute to chronic pain in rodent models. Most of that work focused on somatic pain, but in several studies GI pain was also investigated, and these are discussed in the present review. We have shown that colonic inflammation induced by dinitrobenzene sulfonic acid (DNBS) in mice leads to the activation of SGCs in DRG and increases gap junction-mediated coupling among these cells. This coupling appears to contribute to the hyperexcitability of DRG neurons that innervate the colon. Blocking gap junctions (GJ) in vitro reduced neuronal hyperexcitability induced by inflammation, suggesting that glial GJ participate in SGC-neuron interactions. Moreover, blocking GJ by carbenoxolone and other agents reduces pain behavior. Similar changes in SGCs were also found in the mouse nodose ganglia (NG), which provide sensory innervation to most of the GI tract. Following systemic inflammation, SGCs in these ganglia were activated, and displayed augmented coupling and greater sensitivity to the pain mediator ATP. The contribution of these changes to visceral pain remains to be determined. These results indicate that although visceral pain is unique, it shares basic mechanisms with somatic pain, suggesting that therapeutic approaches to both pain types may be similar. Future research in this field should include additional types of GI injury and also other types of visceral pain.

Identification of Spinal Cord MicroRNA and Gene Signatures in a Model of Chronic Stress-Induced Visceral Hyperalgesia in Rat

Introduction

Animal studies have shown that stress could induce epigenetic and transcriptomic alterations essential in determining the balance between adaptive or maladaptive responses to stress. We tested the hypothesis that chronic stress in rats deregulates coding and non-coding gene expression in the spinal cord, which may underline neuroinflammation and nociceptive changes previously observed in this model.

Methods

Male Wistar rats were exposed to daily stress or handled, for 10 days. At day 11, lumbar spinal segments were collected and processed for mRNA/miRNA isolation followed by expression profiling using Agilent SurePrint Rat Exon and Rat miRNA Microarray platforms. Differentially expressed gene lists were generated using the dChip program. Microarrays were analyzed using the Ingenuity Pathways Analysis (IPA) tool from Ingenuity Systems. Multiple methods were used for the analysis of miRNA-mRNA functional modules. Quantitative real time RT-PCR for Interleukin 6 signal transducer (gp130), the Signal Transducer And Activator Of Transcription 3 (STAT3), glial fibrillary acidic protein and mir-17-5p were performed to confirm levels of expression.

Results

Gene network analysis revealed that stress deregulated different inflammatory (IL-6, JAK/STAT, TNF) and metabolic (PI3K/AKT) signaling pathways. MicroRNA array analysis revealed a signature of 39 deregulated microRNAs in stressed rats. MicroRNA-gene network analysis showed that microRNAs are regulators of two gene networks relevant to inflammatory processes. Specifically, our analysis of miRNA-mRNA functional modules identified miR-17-5p as an important regulator in our model. We verified miR-17-5p increased expression in stress using qPCR and in situ hybridization. In addition, we observed changes in the expression of gp130 and STAT3 (involved in intracellular signaling cascades in response to gp130 activation), both predicted targets for miR-17-5p. A modulatory role of spinal mir17-5p in the modulation of visceral sensitivity was confirmed in vivo.

Conclusion

Using an integrative high throughput approach, our findings suggest a link between miR-17-5p increased expression and gp130/STAT3 activation providing new insight into the possible mechanisms mediating the effect of chronic stress on neuroinflammation in the spinal cord.

Animal models of gastrointestinal and liver diseases. Animal models of visceral pain: pathophysiology, translational relevance, and challenges

Visceral pain describes pain emanating from the thoracic, pelvic, or abdominal organs. In contrast to somatic pain, visceral pain is generally vague, poorly localized, and characterized by hypersensitivity to a stimulus such as organ distension. Animal models have played a pivotal role in our understanding of the mechanisms underlying the pathophysiology of visceral pain. This review focuses on animal models of visceral pain and their translational relevance. In addition, the challenges of using animal models to develop novel therapeutic approaches to treat visceral pain will be discussed.

Role of the potassium chloride cotransporter isoform 2-mediated spinal chloride homeostasis in a rat model of visceral hypersensitivity

Visceral hypersensitivity represents an important hallmark in the pathophysiology of irritable bowel syndrome (IBS), of which the mechanisms remain elusive. The present study was designed to examine whether cation-chloride cotransporter (CCC)-mediated chloride (Cl(-)) homeostasis of the spinal cord is involved in chronic stress-induced visceral hypersensitivity. Chronic visceral hypersensitivity was induced by exposing male Wistar rats to water avoidance stress (WAS). RT-PCR, Western blotting, and immunohistochemistry were used to assess the expression of CCCs in the spinal cord. Patch-clamp recordings were performed on adult spinal cord slices to evaluate Cl(-) homeostasis and Cl(-) extrusion capacity of lamina I neurons. Visceral sensitivity was estimated by measuring the abdominal withdrawal reflex in response to colorectal distension (CRD). After 10 days of WAS exposure, levels of both total protein and the oligomeric form of the K(+)-Cl(-) cotransporter isoform 2 (KCC2), but not Na(+)-K(+)-2Cl(-) transporter isoform 1 (NKCC1), were significantly decreased in the dorsal horn of the lumbosacral spinal cord. The downregulation of KCC2 resulted in a depolarizing shifted equilibrium potential of GABAergic inhibitory postsynaptic current and impaired Cl(-) extrusion capacity in lamina I neurons of the lumbosacral spinal cord from WAS rats. Acute noxious CRD disrupted spinal KCC2 expression and function 2 h after the final distention in sham rats, but not in WAS rats. Pharmacological blockade of KCC2 activity by intrathecal injection of a KCC2 inhibitor [(dihydroindenyl)oxy] alkanoic acid enhanced visceral nociceptive sensitivity in sham rats, but not in WAS rats. These results suggest that KCC2 downregulation-mediated impairment of spinal cord Cl(-) homeostasis may play an important role in chronic stress-induced visceral hypersensitivity.

Petrosal ganglion: a more complex role than originally imagined

The petrosal ganglion (PG) is a peripheral sensory ganglion, composed of pseudomonopolar sensory neurons that innervate the posterior third of the tongue and the carotid sinus and body. According to their electrical properties PG neurons can be ascribed to one of two categories: (i) neurons with action potentials presenting an inflection (hump) on its repolarizing phase and (ii) neurons with fast and brisk action potentials. Although there is some correlation between the electrophysiological properties and the sensory modality of the neurons in some species, no general pattern can be easily recognized. On the other hand, petrosal neurons projecting to the carotid body are activated by several transmitters, with acetylcholine and ATP being the most conspicuous in most species. Petrosal neurons are completely surrounded by a multi-cellular sheet of glial (satellite) cells that prevents the formation of chemical or electrical synapses between neurons. Thus, PG neurons are regarded as mere wires that communicate the periphery (i.e., carotid body) and the central nervous system. However, it has been shown that in other sensory ganglia satellite glial cells and their neighboring neurons can interact, partly by the release of chemical neuro-glio transmitters. This intercellular communication can potentially modulate the excitatory status of sensory neurons and thus the afferent discharge. In this mini review, we will briefly summarize the general properties of PG neurons and the current knowledge about the glial-neuron communication in sensory neurons and how this phenomenon could be important in the chemical sensory processing generated in the carotid body.